1. Field of the Invention
The present invention relates to a drive circuit for ultrasonic motors.
2. Related Background Art
The configuration of an ultrasonic motor is described briefly in conjunction with FIG. 9.
FIG. 9 shows a cross section of an ultrasonic motor. A moving element is formed with a rotor 100-1 and a sliding element 100-2 which are adhered each other. A stator is formed with an elastic element 100-3 and a piezoelectric element 100-4 which are adhered each other. The moving element and stator are pressurized for use by a pressurizing mechanism (not illustrated).
FIG. 10 shows the layout of electrodes in the piezoelectric element 100-4. Electrodes 100-4a and 100-4b are inputs, to which cyclic voltages having a phase difference of .pi./2 are applied. An electrode 100-4c is a common base connected to a ground. An electrode 100-4d does not contribute to the drive of the elastic element, which is used to pick up a monitor voltage.
As mentioned above, an ultrasonic motor consists of a rotor, a sliding element, an elastic element, and a piezoelectric element. Then, driving voltages which are mutually out of phase are input to electrodes 100-4a and 100-4b of the piezoelectric element. This causes the piezoelectric element 100-4 to drive and the elastic element 100-3 to generate a progressive oscillatory wave. Finally, a moving element pressurized to a stator is driven. The configuration and operations of this kind of an ultrasonic motor have already been disclosed in Nikkei Mechanical issued in Feb. 28th, 1983, or the U.S. Pat. No. 4,510,411 owned by the assignee.
For this kind of an ultrasonic motor, a high voltage must be applied to an input. Known prior drive circuits adopt the methods (1) to (3) below to generate a high voltage. That is to say, (1) a sine wave is amplified and applied to an input, (2) a primary voltage is boosted by a transformer and applied to an input, and (3) a square wave is supplied to an input via an inductive element.
In the method (1) that a sine wave is amplified and applied to an input, the configuration of an amplifier becomes complex. A high voltage is required for the power supply to the amplifier. The driving efficiency is low for the supply source of power.
In the method (2) of using a transformer, the transformer must be modified to match an ultrasonic motor. The versatility is poor. In some cases, many elements are required for a circuit which drives the transformer.
In the method (3) that a square wave is supplied to an input via an inductive element, the configuration of a power amplifier which outputs the square wave becomes complex.
FIG. 11 shows an example of a known drive circuit using the aforesaid method (3). An input signal is a square wave having the drive frequency of an ultrasonic motor. In response to the square wave, output transistors 108 and 109 are driven. This makes it necessary to amplify the input signal. Therefore, an amplification circuit for an input signal as shown in FIG. 11 is indispensable. This increases circuit elements in number and makes the circuit configuration more complex. In FIG. 11, 101, 103, and 105 are resistors. 102, 104, 106, 107, and 109 are transistors. 110 and 111 are diodes, and 112, an inductive element.
In this drive circuit, a self-capacitance of an ultrasonic motor input and an inductive element 112 cause a series resonance phenomenon. This boosts the DC supply voltage applied to a drive circuit and changes a square wave or an output voltage wave of an output transistor into a voltage wave approximated to a sine wave. However, an engineer, who belongs to this assignee, failed to set the inductive element value to any value under such a condition that the inductive element value must be determined in such a way that the series resonance frequency caused by the inductive element and self-capacitance will be set to a value approximate to the resonance frequency of the ultrasonic motor.
A drive voltage is determined by the drive frequency of an ultrasonic motor, a self-capacitance, and a DC supply voltage. If the DC power supply consists of batteries and a low-tension cyclic voltage only is applied to a drive circuit, the drive frequency and inductive element value must be high. Under this condition, the supply voltage cannot be boosted enough to be a drive voltage necessary for driving an ultrasonic motor. Therefore, if the aforesaid series resonance phenomenon cannot boost a drive voltage, or if a high-tension drive voltage is required to drive an ultrasonic motor sufficiently, a high-tension power supply is used, or a booster circuit is needed for boosting an output voltage of batteries or other power supply to obtain a high voltage. If the drive voltage must be variable, the voltage value of the DC power supply to the drive circuit must be variable.